Heat-sensitive recording methods have advantages such as that (1) development is unnecessary, (2) when paper is selected as a support, the paper quality may be close to that of an ordinary paper, (3) handling is easy, (4) the colored images of high density is provided, (5) a recording apparatus is inexpensive because of its simple structure, and (6) no noise is generated in recording. Accordingly, recently use of heat-sensitive recording methods has rapidly spread in the fields of facsimile and printers. As such a heat-sensitive recording material used for the method, mainly a heat-sensitive recording material of the leuco color-forming type exhibiting desirable coloration density and color-forming speed has been used. In the heat-sensitive recording material of the leuco color-forming type, however, there is a disadvantage in that the material also tends to form color due to handling after recording, due to heating, or due to adhering of a solvent or the like so that a recording image becomes dirty in appearance.
As a method of preventing such undesired color-formation after thermal recording, a method is known as disclosed by Japanese Patent Application (OPI) Nos. 123086/82 and 125092/82 (corresponding to U.S. Pat. No. 4,411,979), U.S. Pat. No. 3,281,244, and so on (the term "OPI" as used herein refers to a "published unexamined Japanese patent application), in which after thermal recording has been made by using a heat-sensitive recording material composed of a diazo compound, a coupling component and an alkali-generating agent or a coloring assistant, irradiation is performed so as to decompose unreacted diazo compound to stop the color forming. In this method, however, there is a disadvantage in that precoupling gradually progresses during storage (i.e., storage before use) of such a recording material so as to generate undesirable coloration or fog.
As a method of preventing such fog, a method has been proposed in which either one of the coupling component and the coloring assistant is encapsulated by using a nonpolar wax-like material (Japanese Patent Application (OPI) Nos. 44141/82 and 142636/82) or by using a hydrophobic polymeric material (Japanese Patent Application No. 192944/82), so that the material in a capsule is separated from the other components. In these encapsulating methods, however, a microcapsule is formed such that the wax or the polymeric material is dissolved by its solvent and then the color forming component is dissolved or dispersed in the solution and thereafter the solvent is evaporated to be eliminated.
In these encapsulating methods, however, the color forming components are not entirely encapsulated as core materials of the microcapsules, and some of the color forming components are uniformly mixed with the microencapsulating materials, with the result that precoupling is gradually progresses at the wall interfaces of the microcapsules during storage, and therefore a shelf storage property (storability) is not satisfactory. Since a color-forming reaction is generated only when walls of the microcapsules have been thermally fused, the thermal color-forming property is lowered. Further there is such a problem in preparation in that the wax or the polymeric material must be removed after the microcapsules have been formed and therefore the encapsulating methods are not satisfactory.